Update changelog with v2.3.1 release info.
Update changelog with v2.3.1 release info.

Plexxi/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,Plexxi/st2,nzlosh/st2,nzlosh/st2,Plexxi/st2